It is used to provide check whither the data transmitted is correct or not. In this case evenness of 1â€™s is always maintained. For e.g. letter â€˜Aâ€™ is represented as 1000001. In this case number of 1â€™s are 2. To maintain the evenness of 1â€™s 0 should be added on the left side of a number. So that number of 1â€™s is even. If 1â€™s are already odd then 1 should be added on the left side of a number.